An austenitic stainless steel represented by 18Cr-8Ni steel is generally used for superheater or reheater in thermal power plant to ensure high temperature strength. With increase in the steam temperature, however, oxide scale is formed on the inner surface of the steel tube by high temperature steam even in the austenitic stainless steel. Since the austenitic stainless steel generally has large thermal expansion coefficient, the formed oxide scale is exfoliated from the inner surface of the tube by the temperature changes due to the shut-down and start-up cycles of boiler, then the exfoliated scale is deposited at bend sections of the boiler steel tube to induce plugging of the tube. In other cases, the exfoliated scale is scattered into the turbine section to cause erosion of turbine blades.
In recent years, the steam temperature increases more than ever to realize further high efficiency of thermal power plant in view of reduction in Co2 emissions. Accordingly, the improvement in the resistance to high temperature steam oxidation at the inner surface of the steel tube used for these applications, specifically the suppression of oxide scale forming and the prevention of exfoliated oxide scale have become further important technological issues.
As for measures to improve the resistance to steam oxidation for austenitic stainless steels, (1) use of high Cr steel tube, for example 25% Cr steel tube; or (2) use of fine-grained steel tube has been employed. For the case of high Cr steel tube, however, the Ni content is unavoidably increased to form a single austenite phase, resulting in expensive steel tube. For the case of fine-grained steel tube, which improves the resistance to steam oxidation by the refinement of grains in the austenitic stainless steel, the effect is not satisfactory to the increase in the steam temperature. Thus, the fine-grained steel tube cannot prevent the formation of oxide scale, and further encounters to troubles caused by the exfoliation of formed scale.
In JP-A-53-114722, (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Publication”), JP-A-54-138814, JP-A-55-58329, and JP-A-58-39733 disclose methods to improve the resistance to steam oxidation by combining cold-working and heat treatment. Those methods are to improve the resistance to steam oxidation by applying heat treatment after cold-working, enhancing the effect of grain refinement accompanied by the recrystallization of the cold-working portion, and enhancing the effect of oxide film formed during the heat treatment. Similar to the above fine-grained steel tube, however, those methods cannot suppress the formation of oxide scale, and they cannot be expected to maintain the resistance to steam oxidation for a long period.
On the other hands a method to improve the resistance to steam oxidation is provided by applying cold-working only to the inner surface of austenitic stainless steel tube. That is, JP-A-49-135822, “A method for preventing high temperature steam oxidation of steel tube for boiler and for heat exchanger, composed of austenitic stainless steel”, and JP-A-52-8930, “A method for preventing high temperature steam oxidation of austenitic stainless steel” disclose that the work-hardening on the inner surface of the steel tube by shot-blasting has enhanced good resistance to steam oxidation even under the actual plant condition. The steel tube manufactured by the method disclosed in the prior art has shown sufficient resistance to seam oxidation at a steam temperature of 569° C., as described in the examples.
Regarding the steel tube manufactured by the above method, there is a report comparing the mechanical properties between the shot-blasted inner surface of steel tube and the steel tube without treated by shot-blasting in terms of depth of hardened layer being formed by shot-blasting (Takahiro Kanero and Yusuke Minami, “Mechanical Properties of Shot-blasted Stainless Steel Boiler Tubings”, Thermal and Nuclear Power Engineering, Vol. 30, No. 4, pp. 99-105, April, (1979)).
Under the steam conditions in recent years at or higher than 593° C. of steam temperature under ultra supercritical pressures, however, it was confirmed that even the above steel tube with a work-hardening layer on the inner surface of the steel tube by shot-blasting dose not necessarily have the sufficient resistance to high temperature steam oxidation. Consequently, there is a need for an austenitic stainless steel tube with further excellent resistance to high temperature steam oxidation.